As a process to eliminate speckles occurring in an ultrasound image, a filtering process in which a multi-resolution analysis is combined with a non-linear anisotropic diffusion filter is conventionally known.
During a diffusion filtering process that uses a non-linear anisotropic diffusion filter, by applying mutually-different processes to an edge portion and to portions other than the edge portion, it is possible to obtain an image in which the edge is enhanced and from which speckles are eliminated. Further, when a multi-resolution analysis is performed, by sequentially performing processes from a broad-perspective process targeting a low-resolution image to a localized process targeting a high-resolution image, it is possible to perform the diffusion filtering process at a higher speed and more efficiently.
In other words, during the filtering process described above, the diffusion filtering process is applied either to the low-frequency image resulting from the multi-resolution decomposition or to the high-order multi-resolution decomposed image, which is the data output from the level immediately underneath. In this situation, the non-linear anisotropic diffusion filter has a function to enhance the edge. However, when the diffusion filtering process is performed on an image having a low spatial frequency, the structure in a broad perspective is to be enhanced. It is therefore difficult to apply a strong edge enhancement.